1. Field of the Invention
The present invention relates to a semiconductor device using a crystalline silicon film, and to a method of manufacturing the same. More specifically, it is directed to a thin-film transistor using a crystalline silicon film formed on a glass substrate or a quartz substrate, and to a method of manufacturing the same. It also relates to the configuration of a semiconductor device using a crystalline silicon film and to a method of manufacturing the same.
2. Description of the Related Art
Heretofore, there has been known a technology for fabricating a thin-film transistor using a silicon film which is formed on a glass substrate or a quartz substrate by plasma CVD or low-pressure thermal CVD. This technique is utilized not only when a glass substrate or a quartz substrate is used but also when a multi-layer structure is realized in an integrated circuit using a monocrystal silicon wafer.
Studies have been conducted on a technique for using this thin-film transistor in an active matrix type liquid crystal display (LCD) device in particular.
Generally speaking, it is difficult to obtain a monocrystal silicon film by a vapor phase method or deposition method (a technique for forming a monocrystal silicon film on an extremely small region is available but it is not common).
Then, there has been used a technique for forming an amorphous silicon film by plasma CVD or low-pressure thermal CVD and crystallizing the film by heating or irradiating with laser light.
A generally used method for obtaining a crystalline silicon film is a technique for crystallizing an amorphous silicon film formed on a quartz substrate by heat. In this method, heating is carried out at a temperature of 900 to 1,100° C. to modify the amorphous silicon film into a crystalline silicon film.
However, the quartz substrate is expensive and involves a problem when it is used in a liquid crystal display device whose cost reduction has been desired. There is also known a technique in which a glass substrate is used as the substrate. However, since the glass substrate has low heat resistance, it cannot be subjected to the above high-temperature treatment and hence, a required decree of crystallinity cannot be obtained.
The heat resistance temperature of the glass substrate, which differs depending on its type, is in the range of 600 to 750° C. Therefore, it is necessary to obtain a crystalline silicon film having required characteristics through a process at a temperature below that temperature range.
Also, there is known a technique for modifying an amorphous silicon film into a crystalline silicon film by application of laser light. According to this technique, the amorphous silicon film can be modified into a crystalline silicon film without causing thermal damage to the substrate. However, it has problems with the stability of a laser oscillator and uniformity on the illuminated surface. Therefore, it cannot be used on an industrial scale.
As a method for solving the above problem, there is a method in which both a heat treatment and irradiation with laser light are used to increase a process margin. However, when a heat treatment is used, the above treatment temperature problem is encountered, making it difficult to use a glass substrate as well.
As a solution to this problem, there is known a technique disclosed in Laid-open Japanese Patent Application No. Hei 7-074366. This technique uses a metal element for promoting the crystallization of silicon to crystalize an amorphous silicon film at a process temperature of 600° C. or less.
In this technique, there are two crystal growth forms: one is vertical growth (crystal growth in a direction perpendicular to the substrate) which occurs in a region where a metal element is added and the other is horizontal growth (crystal growth in a direction parallel to the substrate) which starts from the region and proceeds around the region.
The vertical growth is characterized in that a crystalline silicon film can be obtained at a lower temperature (crystallizing temperature can be reduced by 50° C.) than heating and that the process is relatively simple. However, since the concentration of the metal element is inevitably high, it has such a problem as the segregation of the metal element.
The segregation of the metal element causes great fluctuations in the characteristic properties of a semiconductor device obtained and an increase in leak current when a thin-film transistor is fabricated.
On the other hand, the horizontal growth is useful because the quality of the resulting film is good and the concentration of the metal element contained therein is low. (this means relatively low). However, when a plurality of horizontal growth regions are selectively formed, they may collide with one another, form a grain boundary or impede the growth of crystals in other regions.
Particularly, since a nickel silicide component is formed in the grain boundary, when the region overlaps with the active layer of a thin-film transistor, the characteristic properties of the thin-film transistor are greatly impaired. It has been discovered that the horizontal growth region which is different from the vertical growth region in the dimension of growth is different in crystal growth form, for example, signal strength indicating each crystal orientation, according to X-ray diffraction measurement.
This causes such problems that, when a large number of thin-film transistors must be formed on a substrate, there are the differences of characteristic properties among the thin-film transistors and that, when a circuit is constructed, an operation failure occurs.
Since a circuit configuration will be integrated more and more highly in the future, the collision or interference between the above horizontal growth regions will become a big problem.
When the direction of growth is different, there will be easily produced differences in the characteristic properties of devices obtained. Therefore, when a circuit having a predetermined function is constructed with a plurality of devices, the difference of growth direction will become a problem to be solved.